How many named Homunculi, each based on a deadly sin, are there in Fullmetal Alchemist?

How to get close to this number

The organising scheme is the answer. These antagonists are named after the seven deadly sins, and that list is fixed by centuries of Christian tradition — so the count is whatever the sins number, not something the author chose freely. Recognising the external scheme a work borrows is the fastest route to any count built on it.

That is the same move as the five elements in the Naruto question on this site: fiction frequently imports a closed set from mythology, religion or folklore, and the size of that set becomes the size of the cast. Once you spot the borrowed structure, the number stops being trivia and becomes deduction.

A caution: the group changes across the story, with members destroyed and in one case replaced by a different character carrying the same name and sin. So the count describes the full roster of named sins rather than how many exist at any moment — and as with several fictional-count questions here, the moment matters as much as the number.